True enough, some sellers do try to milk S&H charges. That repels some buyers, so does the "paypal only" requirement. It's even more common for the seller to get eaten alive with ebay triple-dipping & actual shipping costs. $20 is outrageous to ship a brake pedal, $65 very reasonable for a properly-packaged engine, for examples. Best to consider each transaction individually, on it's own merits. There's no such thing as free shipping...unless you can walk to the seller's location, of course.
At the end of the day, it's up to the seller to do the math. The cheapest way to ship an entire bike is in multiple cartons, after discarding anything that is not reusable. It's still a fair amount of labor to do the teardown & packaging, but shipping cost is usually about half the cost to ship a complete, crated, bike. Crating a bike is no vacation, either and it can be a tricky proposition, with a huge cost range.
If you can find a buyer via CL, it's probably going to be the most efficient venue. Ebay offers a vastly larger audience, and charges mightily for it.